The present invention relates to a method for determining the fundamental frequency or pitch period of a voice signal. More particularly, the present invention relates to a method for determining the fundamental frequency or pitch period of a voice signal utilizing the difference signal, that is generated with the aid of predictors, between the original voice signal and the estimated voice signal produced by the predictor.
Methods are already known which analyze the fundamental vocal chord frequency by means of auto-correlation of the original voice signal. These processes, however, can be strongly interfered with by the influence of formants, so that with low first formants a useful analysis is not possible when using maximum formations in the auto-correlation function. In the same way it is also not possible to produce a pure polarity correlation on the original voice signal.
The known solutions for analyzing voice frequencies from the difference signal of a linear predictive coding (LPC) Vocoder, have, until now, used exclusively the simple maximum-search methods. Such processes, however, work efficiently only with very favorable difference signals. Correlation analyses of the difference signal with full computational accuracy do indeed work effectively from a technical point of view but they necessitate very extensive technical expenditures.
Thus, methods for determining the pitch or fundamental frequency of a voice signal are disclosed, for example, by Man Mohan Sondhi, "New Methods of Pitch Extraction," IEEE Transactions on Audio and Electroacoustics Vol. Au-16, No. 2, June 1968, pages 262-266 and by J. D. Markel, "The SIFT Algorithm for Fundamental Frequency Estimation", IEEE Transactions on Audio and Electroacoustics, Vol. Au-20, No. 5, December 1972, pages 367-377. Both of these articles describe methods which determine the average fundamental frequency or pitch period but need extensive technical expenditures.